Thin film transistor and manufacturing process thereof

ABSTRACT

A thin film transistor including a gate, a gate insulator layer, a channel layer, a source, a drain, and an ohmic contact layer is provided. The gate insulator layer covers the gate; the channel layer is disposed on the gate insulator layer above the gate; the source and the drain are disposed on the channel layer; the ohmic contact layer is disposed between the channel layer and the source and drain. The ohmic contact layer is constituted of a plurality of film layers. As mentioned above, the thin film transistor has an ohmic contact layer constituted by a plurality of film layers. When the thin film transistor is turned off, the current leakage thereof is lowered than that of conventional thin film transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor andmanufacturing process thereof. More particularly, the present inventionrelates to a thin film transistor and manufacturing process thereof, thecurrent leakage thereof is reduced while the thin film transistor isturned off.

2. Description of Related Art

In recent years, it has become more easy and fast for manufacturing thinfilm transistors with the advance in manufacturing technology ofsemiconductor. The appliance of thin film transistors is so widely inmany fields such as chips for computer or mobile phones or thin filmtransistor liquid crystal display (TFT LCD) apparatuses. In TFT LCDapparatuses, the thin film transistors are served as the switches todischarge or charge the storage capacitors.

FIG. 1 is a cross-sectional view of a conventional thin film transistor.The conventional thin film transistor 100 is formed on a glass substrate110. It comprises a gate 120, an insulator layer 130, an amorphous layer140, an N-doped amorphous silicon layer 150, a source 160 and a drain170. A material of the gate 120 is alloy utilized for turning on or offthe electrical channel in the amorphous layer 140. The gate 120 iscoated with the insulator layer 130 which is made of silicon nitride toprevent the gate 120 and the amorphous layer 140 from electricallyconnected. The amorphous layer 140 is disposed on the insulator layer130 to be served as the channel for transmitting electrons. A portion ofthe amorphous layer 140 is covered with the N-doped amorphous siliconlayer 150 to be served as an ohmic contact layer, so as to lower theimpedance between the source 160 and the amorphous layer 140 and betweenthe drain 170 and the amorphous layer 140. The materials of the source160 and the drain 170 are metal and they are disposed on the N-dopedamorphous silicon layer 150.

An electrical channel will be formed on the bottom of the amorphouslayer 140 while a voltage is applied to the gate 120 of the thin filmtransistor 100. The electrons can be flowed to the drain 170 through theelectrical channel while another voltage is also provided to the drain170. The electrical channel formed on the bottom of the amorphous layer140 will be disappeared while no voltage is provided to the gate 120, sothe source 160 and the drain 170 have become open circuit. However,because the conventional thin film transistor 100 is not a flawlessswitch actually, the source 160 and the drain 170 is not an absolutelyopen circuit whether no voltage or a negative voltage is provided to thegate 120. In other words, little current will be still flowed throughthe N-doped amorphous silicon layer 150 and the surface of the amorphouslayer 140 to be the leakage current between the source 160 and the drain170 when the conventional thin film transistor 100 is turned off. Forinstance, if the voltage difference between the source 160 and the drain170 is 10V and the voltage of gate is −6V, the leakage current betweenthe source 160 and the drain 170 is about 10⁻¹¹ and 10⁻¹² amp.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a thin filmtransistor, which has lower current leakage when turned off than that ofthe conventional thin film transistor.

Accordingly, the present invention is directed to a manufacturingprocess of thin film transistor which can fabricate a thin filmtransistor having lower current leakage when turned off than that of theconventional thin film transistor.

The thin film transistor is disclosed in the present invention. Itcomprises a gate, a gate insulator layer, a channel layer, a source, adrain and an ohmic contact layer. The gate is covered by the gateinsulator layer. The channel layer is disposed on the gate insulatorlayer over the gate. The source and drain are disposed on the channellayer. The ohmic contact layer is disposed between the channel layer andthe source and between the channel layer and the drain, wherein theohmic contact layer is constituted by a plurality of film layers.

According to an embodiment of the present invention, wherein the ohmiccontact layer is constituted by a plurality of N-doped amorphous siliconlayers.

According to an embodiment of the present invention, wherein a materialof the channel layer comprises amorphous silicon.

According to an embodiment of the present invention, wherein a materialof the gate comprises metal.

According to an embodiment of the present invention, the first, whereina material of the source and the drain comprises metal.

According to an embodiment of the present invention, wherein a materialof the gate insulator layer is selected from one of silicon oxide,silicon nitride and the combination thereof.

The manufacturing process of thin film transistor is disclosed in thepresent invention. It comprises the following steps. First, a gate isformed on a substrate. And then, a gate insulator layer is formed on thesubstrate to cover the gate. Next, a channel layer is formed on the gateinsulator layer over the gate. Afterwards, an ohmic contact layer isformed on the channel layer, wherein the ohmic contact layer isconstituted by a plurality of film layers. Thereafter, a source and adrain are formed on the ohmic contact layer, and the ohmic contact layerlocated between the source and the drain is removed.

According to an embodiment of the present invention, wherein a method offorming the ohmic contact layer comprises that utilizing a depositionprocess, wherein a deposition power is changed during the depositionprocess so as to form a plurality of film layers to constitute the ohmiccontact layer having a plurality of film layers. And a way to change thedeposition power during the deposition process comprises that turningoff the power at a predetermined time interval during the depositionprocess.

According to an embodiment of the present invention, wherein a method offorming the ohmic contact layer comprises that utilizing a depositionprocess, wherein a deposition pressure condition is changed during thedeposition process condition so as to form a plurality of film layers toconstitute the ohmic contact layer having a plurality of film layers.And a way to change the deposition pressure condition during thedeposition process comprises that changing the pressure at apredetermined time interval during the deposition process, so as tolower the deposition rate.

According to an embodiment of the present invention, wherein the ohmiccontact layer is constituted by a plurality of N-doped amorphous siliconlayers.

As afore-mentioned, in the present invention, a thin film transistor andmanufacturing process thereof, the electron mobility will be reducedbecause the ohmic contact layer is constituted by a plurality of filmlayers. Hence, when the thin film transistor is turned off, the currentleakage thereof is lowered than that of conventional thin filmtransistor. Moreover, when the thin film transistor is turned on, theamount of current of thin film transistor will not change due to theohmic contact layer constituted by a plurality of film layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a cross-sectional view of a conventional thin film transistor.

FIG. 2A to 2E show a manufacturing process of a thin film transistoraccording to one embodiment of the present invention.

FIG. 3A is a top view of a thin film transistor array substrate, whichis a portion of a liquid crystal display apparatus.

FIGS. 3B, 3C and 3D are cross-sectional views of the thin filmtransistor array substrate shown in FIG. 3A taken along lines a-b, c-dand e-f respectively.

DESCRIPTION OF THE EMBODIMENTS

Various specific embodiments of the present invention are disclosedbelow, illustrating examples of various possible implementations of theconcepts of the present invention. The following description is made forthe purpose of illustrating the general principles of the invention andshould not be taken in a limiting sense. The scope of the invention isbest determined by reference to the appended claims.

FIG. 2A to 2E show a manufacturing process of a thin film transistoraccording to one embodiment of the present invention. Referring to FIG.2A, in the present invention, the manufacturing process of a thin filmtransistor includes the following steps. A substrate 210 is providedfirst, wherein the substrate 210 is a glass substrate or anothersubstrate made of other appropriate materials, for example. Next, a gate220 is formed on the substrate 210, and a method of forming the gate 220includes a thin film deposition process, a photolithography process andan etching process.

Referring to FIG. 2B, then a gate insulator layer 230 is formed on thesubstrate 210 and the gate 220 to cover the gate 220, and a method offorming the gate insulator layer 230 comprises a chemical vapordeposition (CVD) process.

Referring to FIG. 2C, after the gate insulator layer 230 has formed, achannel layer 240 is next formed on the gate insulator layer 230 overthe gate 220, and a method of forming the channel layer 240 includes athin film deposition process, a photolithography process and an etchingprocess.

Referring to FIG. 2D, afterwards, an ohmic contact layer 250 is formedon the channel layer 240, wherein the ohmic contact layer 250 isconstituted by a plurality of film layers, for instance. A method offorming the ohmic contact layer 250 comprises a chemical vapordeposition (CVD) process. As above-mentioned, the deposition power canbe changed, for example, by utilizing the chemical vapor deposition(CVD) process to deposit the ohmic contact layer 250, so as to deposit aplurality of film layers on the channel layer 240 to form the ohmiccontact layer 250 having a plurality of film layers. It should be notedthat, a method of changing the deposition power comprises that turningoff the power at a predetermined time interval during the depositionprocess. Additionally, the deposition pressure condition can be changed,for example, by utilizing the chemical vapor deposition (CVD) process todeposit the ohmic contact layer 250, so as to deposit a plurality offilm layers on the channel layer 240 to form the ohmic contact layer 250having a plurality of film layers. Wherein, a method of changing thedeposition pressure condition comprises that changing the pressure at apredetermined time interval during the deposition process, so as tolower the deposition rate.

Referring to FIG. 2E, after the ohmic contact layer 250 is deposited,then a source 260 and a drain 270 are formed on the ohmic contact layer250. Thereafter, the ohmic contact layer 250 located between the source260 and the drain 270 is removed so that the ohmic contact layer 250 adisposed between the source 260 and the channel layer 240 and betweenthe drain 270 and channel layer 240 is remained. Hence, the manufactureof a thin film transistor 200 of the present invention in one embodimentis completed. With regarded to the structure of the thin film transistor200 and the function of the elements thereof will be described asfollowings.

Referring to FIG. 2E, the thin film transistor 200 including a gate 220,a gate insulator layer 230, a channel layer 240, an ohmic contact layer250 a, a source 260, a drain 270.

Located on the bottom of the thin film transistor 200 is the gate 220, amaterial thereof is metal, for example. The gate 220 is mainly utilizedfor controlling the thin film transistor 200 to be turned on or turnedoff. For instance, when a turn on voltage is applied to the gate 220,the thin film transistor 200 is turned on; when a turn off voltage isapplied to the gate 220, the thin film transistor 200 is turned off.

The gate insulator layer 230 is located on the gate 220 to cover thegate 220, wherein a material of the gate insulator layer 230 comprisessilicon oxide, silicon nitride or the combination thereof, for instance.The gate insulator layer 230 enables the gate 220 and the channel layer240 to be electrically insulated.

The channel layer 240 is disposed on the gate insulator layer 230 overthe gate 220, a material of the channel layer 240 can be amorphoussilicon. The channel layer 240 can provide the electrical channel, sothe electrical charge will flow therethrough. When a positive voltage isapplied to the gate 220, the electrical channel will be formed on thebottom of the channel layer 240, so the electrons can flow through theelectrical channel, in other words, the thin film transistor 200 isturned on.

The ohmic contact layer 250 a is disposed on a portion of the channellayer 240 and is located between the source 260 and the channel layer240 and between the drain 270 and channel layer 240. The ohmic contactlayer 250 a comprises a plurality of film layers, a material thereof isN-doped amorphous silicon. The ohmic contact layer 250 a is utilized forlowering the contact impedance between the source 260 and the channellayer 240 and between the drain 270 and channel layer 240. Because theohmic contact layer 250 a comprises a plurality of film layers, ainterface between two neighboring film layers will impede the electronmigration, so that the electron mobility will be reduced slightly.However, when the thin film transistor is turned off, the currentleakage thereof will be reduced significantly thereby than that ofconventional thin film transistor.

The source 260 and the drain 270 are located over the channel layer 240,and the ohmic contact layer 250 a is located between the source 260 andthe channel layer 240 and between the drain 270 and channel layer 240. Amaterial of the source 260 and the drain 270 can be metal to be servedas the conducting wires for transmitting charge.

It should be noted that, the thin film transistor 200 is served as aswitch element when applied to the liquid crystal display apparatus.FIG. 3A is a top view of a thin film transistor array substrate, whichis a portion of a liquid crystal display apparatus. FIGS. 3B, 3C and 3Dare cross-sectional views of the thin film transistor array substrateshown in FIG. 3A taken along lines a-b, c-d and e-f respectively.Referring to FIGS. 3A to 3D, the thin film transistor array substrate300 includes a substrate 300, a plurality of scan lines 320, a pluralityof data lines 330, a plurality of pixel electrodes 340, a plurality ofstorage capacitor lines 350 and a plurality of thin film transistors200. The gate 220 of the thin film transistor 200 is electricallyconnected with the scan lines 320, the source 260 is electricallyconnected with the data lines 330, and the drain 270 is electricallyconnected with the pixel electrodes 340, for example. A storagecapacitor C is formed by the pixel electrodes 340 and the storagecapacitor lines 350. when a turn on voltage is applied to the gate 220via the scan lines 320, the thin film transistor 200 is turned on and anelectrical channel is formed on the bottom of the channel layer 240.When a signal voltage is applied to the source 260 via the data lines330, the charge will flow from the source 260 into the drain 270 by theelectrical channel to charge the pixel electrodes 340. When a turn offvoltage is applied to the gate 220 via the scan lines 320, the thin filmtransistor 200 is turned off and the electrical channel formed on thebottom of the channel layer 240 is disappeared, so the charge is storedin the pixel electrodes 340. Because the ohmic contact layer 250 a ofthe thin film transistor 200 comprises a plurality of film layers, ainterface between two neighboring film layers will impede the electronmigration, so that the electron mobility will be reduced slightly. Butwhen the thin film transistor 200 is turned off, the current leakagethereof will be decreased thereby than that of conventional thin filmtransistor. Therefore, because the leakage current between the source260 and the drain 270 is so tiny when the thin film transistor 200 isturned off, the charge stored in the pixel electrodes 340 will not beleaked rapidly, so as to keep the signal therein.

Table 1 shows that the comparison of the amount of current in theelectrical channel between the thin film transistor having the ohmiccontact layer constituted by a single film layer and another oneconstituted by a plurality of film layers when turned on or turned off.The most of elements of the two thin film transistors are nearly thesame or similar, except for the structure of the ohmic contact layer(but the total thickness of the ohmic contact layers are equal orsimilar).

TABLE 1 carrier mobility in current in current in electrical electricalelectrical initial channel channel when channel when voltage (cm²/turned on (pA) turned off (pA) (V) s · V) thin film 5.13 3.552 3.0930.425 transistor having ohmic contact layer constituted by a single filmlayer thin film 5.05 1.121 2.641 0.399 transistor having ohmic contactlayer constituted by a plurality of film layers

As shown in Table 1, compared to the conventional thin film transistorhaving an ohmic contact layer constituted by a single film layer, whenthe thin film transistors are turned off, the current in the electricalchannel is lowered from 3.552 pA to 1.121 pA in the present invention,thin film transistor which has an ohmic contact layer constituted by aplurality of film layers. Hence, the leakage current in the electricalchannel is reduced significantly when the thin film transistor is turnedoff in the present invention. In addition, compared with theconventional thin film transistor and the thin film transistor of thepresent invention, the current in electrical channel when turned on andthe carrier mobility in electrical channel are closer. In other words,the present invention, the thin film transistor and the manufacturingprocess thereof, can reduce the leakage current thereof substantiallywhen turned off compared to the conventional skill, and the current inelectrical channel when turned on and the carrier mobility in electricalchannel are not influenced so much.

In summary, the present invention, the thin film transistor and themanufacturing process thereof, has advantages as followings:

(1). Compared to the conventional skill, the ohmic contact layer of thethin film transistor according to the present invention is constitutedby a plurality of film layers, although the electron mobility isdecreased slightly due to interfaces between two neighboring filmlayers, the leakage current of the thin film transistor will be loweredsignificantly when turned off.

(2). Because the manufacturing process of the thin film transistor inthe present invention is compatible with the manufacturing processalready existed, the manufacturing process of the thin film transistorin the present invention can be executed without any additionalequipments.

The above description provides a full and complete description of theembodiments of the present invention. Various modifications, alternateconstruction, and equivalent may be made by those skilled in the artwithout changing the scope or spirit of the invention. Accordingly, theabove description and illustrations should not be construed as limitingthe scope of the invention which is defined by the following claims.

1. A thin film transistor, comprising: a gate; a gate insulator layer,covering the gate; a channel layer, disposed on the gate insulator layerover the gate; a source and a drain, disposed on the channel layer; andan ohmic contact layer, disposed between the channel layer and thesource and between the channel layer and the drain, wherein the ohmiccontact layer comprises a plurality of film layers, and at least aninterface configured between a pair of neighboring film layers of theohmic contact layer for impeding electron migration between the pair ofneighboring film layers, wherein the ohmic contact layer is constitutedby a plurality of N-doped amorphous silicon layers.
 2. The thin filmtransistor of claim 1, wherein a material of the channel layer comprisesamorphous silicon.
 3. The thin film transistor of claim 1, wherein amaterial of the gate comprises metal.
 4. The thin film transistor ofclaim 1, wherein a material of the source and the drain comprises metal.5. The thin film transistor of claim 1, wherein a material of the gateinsulator layer is selected from one of silicon oxide, silicon nitrideand the combination thereof.